Sulfur in motor fuels such as gasoline and diesel fuel is an important pollutant. Its concentration has been regulated over the past several years so as not to exceed levels in the range of 500 parts per million (ppm). In order to ensure that the regulated concentration levels are not exceeded, petroleum products are subjected to both laboratory and on-stream analysis during their processing and production. At these concentrations, one of the preferred methods of analysis is x-ray fluorescence spectrometry, described in ASTM methods D 2622 and 4294 incorporated herein by reference, which is well-suited to direct analysis of liquid samples. However, recent government regulations worldwide will reduce the acceptable sulfur contents of gasoline and diesel fuel to below 50 ppm with specific regulatory levels set at 30 and 15 ppm in the next two or three years. It is doubtful whether x-ray fluorescence is sensitive enough to reliably monitor sulfur at 15 ppm, see ASTM Research Report D.02-1456, incorporated herein by reference. There is also a need to monitor total nitrogen, in the ppm range, in liquids such as beverages and fuels. X-Ray fluorescence is not suitable for measuring nitrogen content.
Other more sensitive laboratory methods are xe2x80x9cpyro-UV fluorescencexe2x80x9d for sulfur, according to ASTM method D 5453 incorporated herein by reference, xe2x80x9cpyro-chemiluminescencexe2x80x9d for nitrogen, according to ASTM method D4629 incorporated herein by reference, and xe2x80x9cpyro-electrochemicalxe2x80x9d for either or both sulfur and nitrogen, described in ASTM methods D6366 and 6428 also incorporated by reference herein. In all these methods a small fixed volume of sample is thermally oxidized (xe2x80x9cpyrolyzedxe2x80x9d) and the combustion products are analyzed for SO2 or NO. Ultraviolet fluorescence for SO2 and chemiluminescence for NO both have detection limits of 1 ppb or less, so the sensitivity is good enough to monitor low ppm levels of sulfur or nitrogen in liquids even allowing for the dilution inherent in the pyrolysis step. Similarly, the sensitivity of electrochemical detectors, although not as good as UV fluorescence or chemiluminescence, should be adequate to measure low ppm levels of sulfur and/or nitrogen in liquids, after pyrolysis. However, electrochemical detectors have the great advantages of simplicity and low cost.
Known systems for employing these methods, however, have many drawbacks that are avoided by the present disclosure. In particular, known systems fail to guarantee the quality of the pyrolysis, and as a consequence, reproducible and reliable results may not be obtained and sooting may occur. In employing these analysis methods, a dryer is utilized after pyrolysis to insure the quality of the analysis is not adversely affected by the presence of too much water vapor. However, the dryer arrangement employed in known systems is either costly requiring the use of a separate vacuum pump or may fail to prevent the collapse of the sample dryer inner tube.
The present invention overcomes the drawbacks of known analysis methods by providing reliable and cost-effective on-stream analysis methods and apparatuses for measuring chemical components, including the measurement of total sulfur and nitrogen contents and other components that may be monitored. The present invention accomplishes these objectives by, in certain embodiments, providing reproducible and reliable pyrolysis products and/or by providing an improved dryer design. On-stream analyzers preferably operate automatically and reliably and therefore may include many features, components and improvements that enable the erstwhile laboratory method to function successfully as an on-stream analyzer. Such improvements and additional features are described below.
On-stream analysis for monitoring pollutant levels is of particular importance in many industrial applications. For example the monitoring of sulfur and nitrogen is of concern within the petroleum and beverages industry, however other applications are contemplated by the present disclosure. For simplicity, we refer primarily to monitoring sulfur, although it is contemplated and within the scope of the present invention that the disclosed methods and apparatuses may also be employed for the analysis of other chemical components which may be measured by measuring their pyrolysis products and are capable of detection according to the techniques disclosed herein.
According to the ASTM method, a fixed volume, usually 5-20 microliters, of liquid sample is injected into the pyrolyzer along with an inert carrier gas, usually argon at a flow rate of about 130-160 sccm (standard cubic centimeters per minute) and including some oxygen, about 10-30 sccm. The liquid vaporizes and then reaches the combustion zone where another flow of oxygen, about 450-500 sccm, the xe2x80x9cpyrolysis-gasxe2x80x9d, is introduced and effects complete thermal oxidation at 1050xc2x0 C. The reactor is a quartz tube heated by a tube furnace. The flow rate of liquid sample should never exceed about 4 xcexcl/s (microliters/second), otherwise the combustion process will be starved of oxygen and soot formation (or xe2x80x9csootingxe2x80x9d) will occur, that is, the internal surfaces downstream of the hot zone will be covered with soot. The ASTM methods specify a flow rate of 1 xcexcl/s. The gas output from the pyrolyzer is a mixture of the inert carrier gas (about 20 vol %), unconsumed oxygen (about 60 vol %), carbon dioxide, CO2 (about 10 vol %), water vapor (about 10 vol %) and ppm levels of SO2. The dewpoint is 45-50xc2x0 C., so the gas lines are usually heat traced and/or the water vapor content is reduced to prevent condensation. Water vapor can be reduced without affecting the SO2 content by means of a permeation dryer which operates on the principle of absorption-desorption of water vapor through a membrane (a rapid process having first order chemical kinetics), such as the xe2x80x9cNAFIONxe2x80x9d membrane dryer, commercially available from Perma Pure, Inc. The conditioned gas mixture is then fed to the SO2 detector. A typical 20 xcexcl sample takes some 20 seconds to inject and passes through the pyrolyzer and other gas sample plumbing in about one minute. The SO2 concentration at the detector starts at zero just before the injection, rises to a maximum and then falls off to zero. The rates of rise and fall depend on the various flow rates and gas mixing, and on any molecular exchange reactions that the SO2 undergoes at surfaces it comes into contact with. The detector response ideally follows this same profile. The actual detector response will be less than ideal, so additional broadening of the time profile will occur. In practice, the whole SO2 signal from a given injection will extend over 2-5 minutes. This signal is integrated and is directly proportional to the total amount of sulfur in the original sample. As long as the sample volume remains constant, the SO2 signal is, therefore, proportional to sulfur content of the original sample. xe2x80x9cContinuousxe2x80x9d analysis is accomplished by automating the sample injection procedure.
It is therefore an object of the present disclosure to provide reproducible and reliable pyrolysis byproducts for use in an on-stream analyzer by, in certain embodiments, controlling the volume of a liquid sample dispensed for injection into a pyrolyzer so that it is constant and repeatable. Also, the injection rate is preferably controlled below the upper limit set by xe2x80x9csootingxe2x80x9d and above a lower limit below which the analysis takes too long. In practice, there are closer tolerances set not only on the sample injection rate but also on the flow rates of the input gases. If the detector background signal is negligible, the size of the integrated signal, e.g. SO2 or other chemical signal, would depend only on the total sample volume injected (and its sulfur or other chemical content). Variations in injection rate and in flow rates of the input gases will cause changes in dilution of the SO2 or other components to be measured in the output gas but this would not matter as long as the total signal were to be integrated. In practice, the background signal is not negligible and the amount of time available to complete the integration is limited, so all of it may not be captured. It follows that the dilution of the SO2 or other component in the sample is a factor that may preferably be controlled.
Laboratory instruments employ a microliter syringe, usually motorized, to pump a reproducible volume at a reproducible flow rate. This is not practicable in a process analyzer. Instead, the fixed volume sample can be realized by the use of a sample loop and a two-position (two cycle) multiport valve. The loop is filled from the process stream during one valve cycle and emptied into the pyrolyzer during the second cycle. This is described in U.S. Pat. No. 5,152,963 to Wreyford, incorporated herein by reference. However, specific control of the sample injection rate is apparently not described in U.S. Pat. No. 5,152,963.
A further object of the present disclosure is to offer an improved means for sample injection rate control. In one embodiment the sample injection rate is controlled as follows: Inert gas, such as argon or helium which is readily available, is introduced at a constant pressure, via a pressure regulator, to a flow restrictor such as a length of capillary tubing or a micro-metering valve (an especially preferred embodiment includes use of a micro-metering valve manufactured by Upchurch Scientific that is capable of being turned down to flow rates of less than 1 xcexcl/s). The gas, thus flowing at a constant rate, pushes the liquid sample out of the fixed volume sample loop, through the injector tube and into the pyrolyzer. Since the volume of sample is fixed, the back pressure produced by it is constant until it begins to flow out of the injector. Then the back pressure goes steadily to near zero. The maximum back pressure is below about 0.4 psi for fuel samples with normal viscosity, about 2 cP (centipoise). The regulated pressure should be in the range 5 to 15 psig, preferably about 8 psi so the relative change in back pressure is less than 10% and is repeatable from sample to sample. The flow rate is calibrated prior to operation using a bubblemeter and stopwatch or by timing the appearance and development of the liquid sample drop at the injector tip with the injector out of the pyrolyzer. This assembly provides an injection rate constant to about 10-20%.
Other methods of controlled injection operate by positive displacement of the liquid sample using a piston. Apart from the abovementioned microliter syringe, operated automatically by a syringe pump or manually, one could employ a piston pump or dispenser such as the FMI Model PiP00SKY. Each of these alternate methods are contemplated by the present invention.
U.S. Pat. No. 5,152,963 and the ASTM methods employ standard laboratory rotometers (floating ball flow meters) with metering valves for flow control of the carrier gas and oxygen. At the flow rates stated above, a 10% change in each of the input gases will produce the following changes in output sample flow rate and resulting equal changes in SO2 or NO content through dilution: argon carrier gas, 2%; oxygen pyrolysis gas, 7% and oxygen carrier gas, 0.4%. Errors of this magnitude will show up in the measured sulfur or nitrogen content. An additional object of the present disclosure is to measure each gas flow with an electronic flow meter. The flow data are fed to the Programmable Logic Controller, PLC, and are available to make corrections as necessary to the SO2 reading. Also contemplated is the use of flow alarms to indicate failure of any gas flow and to greatly reduce the danger of catastrophic equipment failure due, for example, to xe2x80x9csootingxe2x80x9d. Furthermore, the output gas flow may also be monitored. This enables flow balance calculations to be made with the resulting ability to detect gas leaks, for example. Finally, the quality of the pyrolysis can be checked by monitoring small fluctuations in the output gas flow that occur during pyrolysis.
Other alternate or preferred embodiments of the present disclosure include one or more of the following. (1) The pyrolysis furnace may be enclosed in a sealed container whose outside surface temperature never exceeds 200xc2x0 C. (this is the xe2x80x9csurface temperature classificationxe2x80x9d, T3, defined for hazardous locations relevant to our application). Thus, if the enclosure purge fails and the electrical power is consequently cut off, the furnace will still be at over 1000xc2x0 C. and will take hours to cool down. In a hazardous location, such as an oil refinery, explosive vapors could reach the furnace and cause an explosion even when the electrical power is off. Sealing the furnace is an acceptable method for maintaining equipment safety. (2) A combustible gas sensor may be installed in the enclosure to indicate and provide an alarm for any leak of possibly explosive vapors into this enclosure. (3) The heat trace element for the gas sample stream may comprise a self-limiting cable for extra safety. Such a cable, for example, Omega, Cat# SRL5-2, comprises two parallel conductors separated by a partially conducting polymer. As the temperature increases the resistance between the conductors increases, reducing the heating current. The upper temperature may be limited to about 66xc2x0 C. (4) The gas sample stream dryer may be operated in a novel manner to avoid problems due to back pressure caused by dryer tube collapse (see detailed description, later). (5) The SO2 or NO detector may be an electrochemical cell, not a UV fluorescence analyzer. (6) A humidifier may be included just upstream of the electrochemical cell to prevent the relative humidity of the gas filling too low between liquid sample injections, when only dry argon and oxygen are flowing, or when calibration gas is being introduced from a compressed gas cylinder. (7) The relative humidity of the gas sample stream may be monitored and alarm signals available if the relative humidity exceeds a preset upper or lower limit. (8) Liquid sample and calibration sample routing outside the instrument enclosures may be performed by pneumatically operated valves, controlled from within the enclosures, which are intrinsically safe. (9) The calibration liquid samples may be pumped using air pressure, also intrinsically safe. (10) The flow rate of calibration liquid may be preset and controlled separately from that of the process fluid in order to conserve expensive calibration liquid and lengthen the time interval required between refilling the calibration reservoirs. (11) The injector may be mounted so that its tip penetrates the pyrolyzer just far enough to where the pyrolyzer temperature is high enough to smoothly volatilize the liquid sample but not so high as to cause chemical breakdown inside the injector.
One preferred embodiment of the disclosed on-stream analyzer which measures the concentration of a substance in a fluid sample includes a sample injector, a thermal oxidizer, a sample conditioner, a detector, and a programmable logic controller. The sample injector injects the sample at a preset and controlled rate, the sample injector optionally includes a pressure regulator coupled to a flow restrictor, effective to restrict the flow rate of the sample, thereby slowing, reducing, or eliminating the sample flow. Examples of preferable flow restrictors include a metering valve or a length of capillary tubing having a diameter of about 0.001 to about 0.020, preferably 0.004 inches, to control the rate of sample flow. The thermal oxidizer may be enclosed within a container, designed to have an outer surface temperature that does not exceed the lower explosive temperature of the sample being analyzed and to prevent ingress of vapors of the sample being analyzed, and has a tube furnace and a pyrolysis tube, connected to the sample injector so that the sample is injected into the pyrolysis tube, volatilized, and a carrier gas, i.e. an inert gas which is preferably argon, helium, or mixtures thereof and optionally containing an admixture of oxygen, and a pyrolysis gas, preferably oxygen, are introduced at a preset, controlled rate, under oxidation conditions. Metering valves are utilized to control the flow rates of the carrier and pyrolysis gases and electronic flowmeters, connected to the programmable logic controller, are employed to measure the flow rates of the carrier and pyrolysis gases as well as the total flow rate of the gas mixture. The electronic flow meters may optionally include an alarm that indicates a flow rate of either carrier or pyrolysis gases that falls outside a preselected range. Oxidation conditions may include a temperature ranging from about 1000xc2x0 C. to about 1200xc2x0 C. and preferably about 1050xc2x0 C. The sample conditioner, connected to and located downstream from the thermal oxidizer, controls the conditions of the resulting gas mixture. The sample conditioner preferably includes one or more of the following, a filter, a dryer, and/or a heat trace element. The filter is preferably made of a chemically inert material, such as polytetrafluorethylene, and prevents solid particles or liquid drops from passing downstream thereby preventing contamination of downstream components and/or surfaces. The heat trace element is preferably composed of self-limiting electrical heating wires that are effective to maintain the gas stream above its dewpoint from the pyrolysis tube to the dryer. The dryer preferably contains two concentric tubes, an inner tube and an outer tube. The inner tube composed of a membrane for transferring water vapor, preferably the membrane contains active groups for effectuating the water transfer and the active groups are preferably sulfonic acid groups. The outer tube is composed of an inert material that is preferably a fluoropolymer or 316 stainless steel. It is preferable for the dryer to be configured so that a dry purge gas passes through the inner tube and the sample gas is directed through the annular space between the outer tube and the inner tube. Further, the dryer may optionally contain a restrictor, connected to the inner tube to maintain positive pressure within the dryer. The restrictor preferably includes an orifice having a diameter of about 0.001 inches to about 0.05 inches, preferably 0.0122 inches. The dryer may optionally include metering valves to control the purge gas flow and electronic flowmeters, connected to the programmable logic controller, to measure the purge gas flow. The electronic flow meters preferably include an alarm to indicate a purge gas flow that falls outside a preselected range. The detector is connected to the sample conditioner and measures the concentration of oxidized substance contained within the resulting gas mixture. The detector preferably includes an assembly of one or more electrochemical cells where the electrochemical cells are preferably sensitive to SO2 or NO and may contain an arrangement of electrochemical cells capable of detecting both SO2 and NO. The programmable logic controller, connected to the detector, calculates the concentration of oxidized substance contained within the gas mixture and consequently the concentration of substance in the original fluid sample.
Another preferable feature that may be optionally included is a combustible gas sensor to indicate the presence of combustible gas vapors. In addition, it is preferable to utilize a humidifier, located upstream from the detector, to maintain the relative humidity level between about 15 to about 90%. The humidifier may optionally include a relative humidity sensor with an alarm to indicate a relative humidity level that falls outside a preselected range.
It is also preferable to utilize non-electric valves to route the fluid sample to the thermal oxidizer, the non-electric valves are preferably pneumatically operated and therefore intrinsically safe. The analyzer also preferably includes a fluid sample outlet and a non-electric flow indicator to indicate the flow rate of the sample through the fluid sample outlet.
Another optional feature is one or more pneumatic valves for introducing one or more calibration fluids, contained in one or more calibration sample reservoirs (storage vessels), to the injector by means of non-electrical valves, more preferably pneumatically operated valves. The flow rate of the calibration fluids may optionally be controlled with metering valves. Also, a preferred embodiment may include solenoid valves, controlled by the programmable logic controller, to effectuate flow of the calibration gas directly to the detector, bypassing the thermal oxidizer and sample conditioner. Further, pneumatically-actuated pressure switches to send signals, via the solenoid valves, to the programmable logic controller to indicate low gas pressure is also preferred.
Another embodiment of the present disclosure involves a method for drying a gas mixture including the steps of providing a gas mixture to be dried, passing a purge gas through an inner tube, feeding the gas mixture through the annular space between the inner tube and an outer tube, restricting the purge gas flow, and capturing the dried gas mixture. The inner tube is comprised of a membrane for transferring water and preferably the membrane contains sulfonic acid groups. The outer tube is comprised of an inert material, preferably a fluoropolymer or 316 stainless steel. It is preferable that the restricting of the purge gas flow is accomplished with a restrictor having an orifice, between about 0.001 to about 0.05 inches in diameter, more preferably 0.0122 inches, to maintain positive pressure.